Selective and stable iso-butene production over highly dispersed VO x species on SiO2 supports via combining oxidative and non-oxidative iso-butane dehydrogenation

Article abstract of DOI:10.1039/c0cc00847h

For the first time, we demonstrated that combining oxidative and non-oxidative dehydrogenation of iso-butane in one reactor system is an attractive approach for iso-butene and hydrogen production. Iso-butene yields above 40% at iso-butene selectivity abov

Full text of DOI:10.1039/c0cc00847h

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Selective and stable iso-butene production over highly dispersed VOx
species on SiO2 supports via combining oxidative and non-oxidative
iso-butane dehydrogenationw
Olga Ovsitser and Evgenii V. Kondratenko*
Received 9th April 2010, Accepted 20th May 2010
First published as an Advance Article on the web 8th June 2010
DOI: 10.1039/c0cc00847h
For the first time, we demonstrated that combining oxidative and
non-oxidative dehydrogenation of iso-butane in one reactor
system is an attractive approach for iso-butene and hydrogen
production. Iso-butene yields above 40% at iso-butene selectivity
above 80% and COx selectivity below 2% were achieved over
SiO2-supported highly-dispersed VOx species during at least
20 h on-stream at 833 K.
to combine ODH and DH processes in one reactor system in
order to suppress consecutive iso-butene oxidation to COx at
high iso-butane conversions. Moreover, the presence of small
amounts of O2 helps at least partially to in situ burn carbon
deposits from the catalyst surface resulting in an improved
time-on-stream stability. In addition, such coupling of
exothermic and endothermic reactions can enable thermo
neutral or slightly exothermic reactor operation. Attempts to
improve DH processes by O2 addition have already been
described in the literature.10,11 However, they differ from our
approach. For example, Grasselli et al.10 used two separate
reactors loaded with Pt- and Bi-containing materials for the
non-oxidative dehydrogenation of propane and hydrogen
oxidation, respectively. Iglesia et al.11 suggested alkane
dehydrogenation over a Pt-containing zeolite in one reactor
with staged introduction of O2 for H2 removal. In contrast to
these previous approaches, we intended to prevent H2 burning,
because it is also an important reaction product.
A vital challenge in modern catalysis is functionalization of
cheap and in future abundant C2–C4 alkanes from natural gas
towards corresponding olefins and oxygenates. These products
are important building blocks of the chemical industry and are
currently produced from depleting oil-derived feeds. There are
two direct approaches for olefin production from alkanes, i.e.
their non-oxidative (DH) and oxidative (ODH) dehydrogena-
tion. The former process is commercialised. However, it is
thermodynamically constrained and suffers from rapid catalyst
deactivation. The oxidative dehydrogenation of alkanes is free
of these limitations. Its main drawback is low olefin selectivity
at high alkane conversions due to fast consecutive oxidation of
the target olefin to carbon oxides. Numerous catalytic systems
have been studied for the DH and ODH reactions.1–3
Vanadium-based catalysts belong to the best performing
materials in the oxidative dehydrogenation of light hydro-
carbons. They were also applied for the non-oxidative
dehydrogenation of C3–C4 alkanes.4–8 However, their activity
strongly decreases with time-on-stream under oxygen-free
conditions. Thus, better catalysts or modes of reactor
operation are required to allow for the stable and selective
production of olefins at industrially attractive levels of alkane
conversion.
The catalysts used in this study were prepared by impreg-
nating silica supports (MCM-41 and amorphous SiO2) with
vanadyl acetyl acetonate dissolved in toluene followed by
drying at 400 K for 12 h and calcining at 823 K for 12 h.12
The samples are denoted here by their amount of vanadium
loading (e.g. sample with V loading of 5.3 wt%, designated as
VOx(5)/MCM-41). Details of thorough catalysts characteriza-
tion by TEM, TPR, in situ UV/Vis, and in situ Raman
spectroscopy have been reported previously.12–14
Catalytic tests were performed in a fixed-bed quartz reactor
at atmospheric pressure and 773–833 K using 0.1–0.6 g of
catalyst. Feed concentration of iso-butane and oxygen in neon
was varied from 15 to 80 vol.% and from 0 to 20 vol.%,
respectively. The contact time was in the range of
0.03–15.0 g s mlÀ1. The feed components and the products
were analysed by on-line GC (HP 5890-II) using a Poraplot Q
column and a Molsieve 5 column; Ar was used as carrier gas
for GC analysis of H2.
We report, for the first time, the superior performance of
silica-supported vanadium-based materials in the conversion
of iso-butane to iso-butene. Iso-butene yields above 40% at
iso-butene selectivity above 80% and time-on-stream stability
are comparable with or even superior to the highest values
reported in patent and open literature over Pt- and
CrOx-containing catalysts.2,9 This was possible due to
operating under oxygen-lean conditions (co-feeding iso-C4H10
and small amounts of O2). The idea behind this approach was
The catalytic performance of VOx(5)/MCM-41 in iso-
butane functionalization at different iso-butane/oxygen ratios,
temperatures and contact times is summarized in Table 1 and
Fig. 1. The results presented in the Table clearly demonstrate
that operating with a stoichiometric ratio of iso-C4H10/O2 = 2
and a short contact time is not suitable for selective formation
of iso-butene; the iso-butene selectivity of 20% was achieved
at iso-butane conversion of only 9%. The selectivity and the
conversion increased to approximately 80 and 50%, respec-
tively, when the ratio of iso-C4H10/O2 was significantly higher
(410) than the stoichiometric one. Moreover, the selectivity
Leibniz-Institut fur Katalyse e. V. an der Universitat Rostock,
¨
¨
Albert-Einstein-Str. 29a, D-18059 Rostock, Germany.
E-mail: Evgenii.Kondratenko@catalysis.de;
Fax: +49(381)1281-51290; Tel: +49(381)1281-290
w Electronic supplementary information (ESI) available: Calculation
of conversion, selectivity and yield; Fig. S1. See DOI: 10.1039/c0cc00847h
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iso-butene yield of 31% was obtained.15 Furthermore, the
COx selectivity in this previous study was above 30%.
The space time yield of iso-butene (STY(iso-C4H8)) under
the oxygen-lean conditions in our study was between 0.24 and
À1
1 kgiso-C H kgcat
h
À1, which is also attractive from an
4
8
industrial point of view. An additional advantage of such an
operation is co-production of hydrogen. The ratio of
H2/iso-C4H8 was slightly lower than one.
The selectivity–conversion plot in Fig. 1 further highlights
the importance of the oxygen-lean reactor operation for
selective conversion of iso-butane to iso-butene. When
the dehydrogenation reaction was performed using an
iso-C4H10/O2 = 2 feed, the iso-butene selectivity strongly
decreased with increasing iso-butane conversion due to
consecutive oxidation of iso-butene to carbon oxides. These
non-selective reaction pathways were practically suppressed
under oxygen-lean conditions; the iso-butene selectivity
slightly dropped from 95 to 80% with an increase in the
iso-butane conversion from 20 to 55%. The decrease in the
selectivity was mainly due to the conversion of iso-butane to
1-, 2-butenes and propene, but not due to the formation of
COx (Table 1).
Fig. 1 Selectivity–conversion relationship in iso-butane dehydro-
genation over VOx(5)/MCM-41.
towards carbon oxides dropped below 2%. In order to achieve
such high degrees of iso-butane conversion, the iso-butane
dehydrogenation with oxygen-lean feeds was performed at a
slightly higher temperature (833 K) and longer contact times
(1–15 g s mlÀ1) compared to the dehydrogenation with the
ratio of iso-C4H10/O2 = 2. The high temperature and the long
contact time are not the only key parameters for selective
iso-butane conversion to iso-butene. For example, when the
ODH reaction was performed at 833 K with t of 2.5 g s mlÀ1
using the stoichiometric feed, the COx selectivity was 32%,
while the iso-C4H8 selectivity did not exceed 60% at 50%
iso-C4H10 conversion (Table 1). In other words, the ratio of
iso-C4H8 to O2 is crucial for the iso-C4H10 functionalization.
In order to prove if iso-butene was formed catalytically under
these conditions, we tested the iso-butane dehydrogenation
over fused SiO2 at 833 K and a contact time of 5 g s mlÀ1. The
degree of iso-butane conversion did not exceed 0.3% without
the catalyst.
In order to elucidate the time-on-stream performance of
oxygen-lean dehydrogenation of iso-butane to iso-butene, we
performed long term catalytic tests over VOx(5)/MCM-41,
VOx(1.3)/SiO2 and VOx(10)/MCM-41. The catalysts differ in
the nature of VOx species; highly-dispersed VOx species are the
main surface species over VOx(5)/MCM-41, and VOx(1.3)/SiO2,
while crystalline nano-sized V2O5 aggregates dominate in
VOx(10)/MCM-41.12 Fig. 2 compares the iso-butene yield
obtained over these catalysts over 20 h on-stream. The
VOx(10)/MCM-41 material with crystalline nano-sized V2O5
lost approximately a half of initial iso-butene yield during 20 h
on-stream mainly due to decreasing the conversion of
iso-butane. Contrarily, the highly-dispersed VOx species
showed exceptionally stable time-on-stream performance.
Such stable operation is due to the very low formation of
carbon deposits on the catalyst surface. For example, the
amount of carbon deposits formed over the VOx(1.3)/SiO2
catalyst after 21 h on-stream was only 0.0027 g per 1 g of
catalyst, i.e. approximately 0.3 wt.% of the catalyst amount.
The stable time-on-stream performance is an important
and significant feature of oxygen-lean operation over
silica-supported vanadium-based catalysts compared to the
The high iso-butene yield achieved in the study is an
important difference compared to previously reported results
on the oxidative dehydrogenation of iso-butane over a
vanadium/antimony-based catalyst, where the maximum
Table 1 Catalytic performance of VOx(5)/MCM-41 in iso-butane dehydrogenation under different reaction conditions after 2 h on stream.
Conversion, yield, and selectivity were feed-based calculated according to equations 1–4 in the supporting information
Feed/vol.%
Selectivity/%
STY(iso-C4H8)/
Y(iso-C4H8)/% kgiso-C H kgcat hÀ1
a
W/F/g s mlÀ1 T/K X(iso-C4H10)/%
À1
iso-C4H10
O2
iso-C4H8 n-C4H8
C3H6 COx
4
8
Oxygen-lean conditions
15
15
40
80
0
2.5
2.5
5.0
1.6
833
833
833
833
53
54
47
28
81
85
85
86
15.3
9.9
8.1
11.6
2.5
2.5
1.7
1.3
1.0
2.0
1.3
0.7
43
46
40
24
0.24
0.26
0.30
1.1
1.0
1.5
0.5
Oxygen-rich conditions
15
40
7.5
20
2.5
0.04
833
773
50
9
57
20
5.2
2.1
2.8
2.5
32
70
28
1.8
0.16
1.80
a n-C4H8 = 1-C4H8 + 2-cis-C4H8 + 2-trans-C4H8
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with participation of lattice oxygen of VOx species, and ii)
the non-oxidative dehydrogenation of iso-butane to iso-butene
and hydrogen. Presence of oxygen and water (product of
oxidative conversion of iso-butane/iso-butene) is important
to minimize the formation of carbon deposits. The relative
contribution of the oxidative and non-oxidative processes to
the overall olefin production depends on reaction temperature,
contact time and oxygen content. Since the non-oxidative
reaction is significantly slower and more thermodynamically
limited than the oxidative one, high operation temperatures
are required to achieve industrially attractive space time yields.
However, the operating temperature should not be too high in
order to avoid further pyrolysis reactions leading finally to
carbon and hydrogen. Current investigations are aimed
at increasing the activity of highly dispersed VOx species
by catalyst design and improving long-term stability by
co-feeding O2, CO2, and H2O.
In conclusion, we successfully demonstrated that oxygen-
lean dehydrogenation of alkanes is an effective method for
selective and stable production of olefins and hydrogen.
Iso-butene selectivity above 80% and COx selectivity below
2% were achieved at iso-butane conversions above 50%. This
appears to be attractive for industrial applications, and inter-
esting from a fundamental point of view.
Fig. 2 Iso-butene yield as a function of time-on-stream at 833 K over
VOx(5)/MCM-41 (W/F = 5 g s mlÀ1), VOx(1.3)/SiO2 (W/F =
15 g s mlÀ1) and VOx(10)/MCM-41 (W/F = 15 g s mlÀ1). Reaction
feed: 15 vol.% iso-C4H10, 1 vol.% O2 in Ne.
Support by Deutsche Forschungsgemeinschaft (DFG) within
the frame of the competence network (Sonderforschungsbereich
546) ‘‘Structure, dynamics and reactivity of transition metal
oxide aggregates’’ is greatly appreciated.
non-oxidative dehydrogenation of butanes over VOx/Al2O3, in
which the catalytic activity strongly decreased within the first
30–60 min on-stream due to a significantly higher coke
formation.4–6 Compared to the VOx(1.3)/SiO2 catalyst used
in our study, ten times higher amount of carbon deposits was
determined over VOx/Al2O3 already after 15 min on-stream.6
It is important to stress that the significantly higher coke
formation over VOx/Al2O3 than over the VOx(5)/MCM-41
and VOx(1.3)/SiO2 materials can not be explained just by the
higher activity of VOx/Al2O3, because the authors in5,6 used
similar contact times (W/F = 5 g s mlÀ1) and ever more
diluted butane feeds (10 vol.% iso-butane).5,6 The high time-
on-stream stability of our catalysts is related to: i) the
difference in the acid–base properties of Al2O3 and SiO2,
and ii) the mode of operation, i.e. co-feeding small amounts
of O2 to iso-C4H10. Under such conditions, precursors of
stable carbon deposits can be at least partially oxidized by
oxygen. The results of temperature-programmed oxidation
tests in Fig. S1 in the supporting informationw maintain this
conclusion. Carbon deposits formed during iso-butane
dehydrogenation over VOx(1.3)/SiO2 at 833 K over 21 h
on-stream were easily oxidized. CO2 was the only product
with a maximal formation at 723 K.
Notes and references
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From a mechanistic point of view the dehydrogenation of
iso-butane over vanadium-containing catalytic materials
under oxygen-lean conditions can be explained as follows.
At least two processes contribute to the formation of
iso-butene: i) the oxidative dehydrogenation of iso-butane
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4976 | Chem. Commun., 2010, 46, 4974–4976